The corrosion of steel in reinforced concrete structures, particularly in northern latitudes where de-icing salts are widely employed, is a constant and increasing problem. The primary cause of such corrosion is the contamination of the concrete by chloride ions resulting from such de-icing salts either applied directly to the surface of the structure as in the case of bridge decks and roadways, or brought in on automobiles as in the case of garages. Studies performed by the National Bureau of Standards on the corrosion of steel in concrete indicate that the corrosion products can occupy over 2.2 times the original volume of steel causing severe internal pressures up to 330 Kg/cm.sup.2. These pressures result in the concrete spalling and cracking. Where corrosion cells are provided on the reinforcing steel, high chloride ion concentration in the concrete deck or floor slab results in depolarization of the corrosion cells, produces an increased corrosion rate.
To fight this corrosion problem, there are generally three choices. The first is to coat the surface of the structure or deck to minimize penetration by the salts. The second is to modify the concrete to make it more resistant to chloride penetration, and the third is to permit the chloride penetration of the concrete but protect the steel as, for example, by cathodic protection.
The first two methods outlined above have many drawbacks and are not always available in existing structures. Both overlays and modification of the concrete are expensive. Coatings or overlays simply preclude further chloride penetration and if they are of any thickness they can create deadload problems. One of the principal drawbacks of overlays or coatings is that they preclude further visual inspection of the surface. In the area of protecting the steel itself, for new structures, coated or galvanized reinforcing bars may be employed. Coated reinforcing bar, while useful in new structures, is useful in existing structures only with substantial reconstruction.
Cathodic protection has been used for many years to combat corrosion of buried pipelines and structures in salt water environments. It has only recently been utilized in reinforced concrete structures.
In steel reinforced concrete structures, it is recognized that steel corrosion is the result of an electrical current flowing from one point on the steel to another. Such corrosion current is simply enhanced by moisture and salt contamination. If the flow of current is interrupted, the corrosion stops. Cathodic protection applies an external direct current to the steel in sufficient quantity to reverse or counteract the corrosive current.
To applicant's knowledge, prior cathodic protection systems for steel reinforced concrete decks or structures have utilized small point or pancake type anodes of either high silicon cast iron such as Durion, or graphite anode material. Pancake anodes, for example, are disc shape approximately 30.48 cm. in diameter and approximately 3.81 cm. thick. The leads or connections to the anodes extend centrally through the peripheral wall or edge and are tinned into a cored hole approximately 7.5 cm deep. Such anodes are typically strategically placed on the deck and may be secured thereto by epoxy cement. They are then generally covered by a conductive overlay, consisting of a 15 to 25% asphaltic material mixed with approximately 75 to 85% coke breeze material. The purpose of the conductive overlay is to distribute the current discharge from the anode out over the entire deck or bridge surface area and then to cause it to flow evenly down through the concrete to the reinforcing rod or steel to eliminate the corrosion on the steel. Typically, an additional asphaltic overlay is employed as a traffic wear surface. Accordingly, the entire overlay may typically be 5 or more centimeters in thickness. Such previous systems, although quite expensive, are in some cases suitable but do in many cases create problems.
For example, the conductive overlay must be put on in a relatively thick layer which adds a considerable deadload to the structure. In many cases, this additional weight may exceed the design limitations of the structure making the approach either impractical or causing reduced loading. In addition, as indicated above, an overlay precludes further visual inspection of the top surface of the deck or structure.
The small point or pancake type anodes employed which themselves have a fairly high corrosion rate, are of a fairly brittle material which when subjected to the traffic loads through the relatively soft overlays are subject to cracking or breakage during the life of the system. Also, the plastic insulated lead wires and their connection to the fragile anodes are particularly susceptible to shear breakage and to wear damage. The plastic insulated lead wires used to connect the anodes to the DC power supplies are often damaged in installation when the overlay is installed since the overlay is applied at a very hot temperature, 176.degree. C. or higher being typical. Moreover, in a typical system, such pancake or point type anodes are generally zoned with the anodes in each zone being connected in series. Thus not only is an inordinate length of lead wire required, but each anode usually has two connections thus increasing the susceptibility to failure.